Preparation of arylphosphonic acids



PREPARATION OF ARYLPHOSPHONIC ACIDS Tsai H. Chao, Somerville, Hans Z.Lecher, Plainfield, and Ruth A. Greenwood, Somerville, N. 1., assignorsto American Cyanamid Company, New York, N. Y., a corporation of Maine NoDrawing. Application March 25, 1955 Serial No. 496,934

15 Claims. (Cl. 260-500) This invention relates to a new process ofpreparing arylphosphonic acids. It also relates to arylthionophosphinesulfides which are new compounds and are inter mediates in said process.

A number of arylphosphonic acids having the formula ArPO(OI-I) in whichAr is an aryl radical have been known but until recently they have beenlaboratory curiosities because the only way of preparing them was byround-about and expensive processes which precluded their practicalutilization.

The first process which permitted production of arylphosphonicacids'with sufiicient efficiency to make them practical on a commercialscale is described and claimed in Patent No. 2,717,906, issued September13, 1955, on the copending application of Lecher, Chao, and Whitehouse,Serial No. 345,264, filed March 27, 1953, which is acontinuation-in-part of an earlier application, Serial No. 286,614,filed May 7, 1952, and now abandoned. According to this processcarbocyclic aromatic compounds -free from polar groups capable ofreacting with phosphoric anhydridewere phosphonated with hexagonalphosphoric anhydride within a temperature range of from 250325 C.Reaction products were obtained which on hydrolysis with water yieldedarylphosphonic acids. The reaction products were also convertible intoarylphosphonic dichlorides by reaction with phosphorus pentachloride asdescribed and claimed in the copending application of Greenwood, Scaleraand Lecher, Serial No. 357,368, filed May 25, 1953, now U. S. Patent No.2,814,645, dated November 26, 1957.

While this new process gave satisfactory yields of arylphosphonic acidsbased on the real usage of the aromatic compounds (an excess of thearomatic compound was used but could be recovered and re-used), theprocess still possessed certain drawbacks from the cost standpoint. Themost serious factor is that only one or two at most of the phosphorusatoms of the phosphoric anhydride P reacted with the aromatic compound.The remaining phosphorus was either lost as phosphoric acid in the caseof water hydrolysis or had to be removed as phosphorus oxychloride inthe case of production of phosphonic dichlorides. In both cases thisrepresented a waste of raw materials and added to the cost of the finalarylphosphonic acids.

Another drawback was that with certain aromatic compounds. such asnaphthalene or alkyl derivatives of benzene the phosphoric anhydridecatalyzed condensation reactions forming by-products which complicatedthe isolation of the phosphonic acids and, of course, lowered theyields.

The present invention avoids the disadvantages of the prior process byusing phosphorus pentasulfide or its chemical equivalents instead of thehexagonal phosphoric anhydride. Reactions of phosphorus pentasulfidewith hydrocarbons, including aromatic hydrocarbons, are not new in theart. However, the resulting products have been mixtures containing notonly thionophosphine sulfides but also lower sulfides of phosphorus andother by- States Patent Patented Aug. 112, 1958 ice "' tergentproperties to the oils. Some of these products have also been proposedas flotation agents.

The prior art reaction products of aromatic hydrocarbons with phosphoruspentasulfide have never been completely hydrolyzed to form phosphonicacids. Some such crude reaction products have been blown with steam ornitrogen toremove malodorous thiocompounds which are formed asby-products. However, the prior art emphasized that after this treatmentthe products still contained substantial amounts of sulfur, that is tosay they were not completely hydrolyzed.

According to the present invention, phosphorus pentasulfide is heatedwith an aromatic compound at temperatures of -250 C., the aromaticcompound being free from polar substituents capable of reacting withphosphorus sulfides. Hydrogen sulfide is evolved and anarylthionophosphine sulfide is formed. Thelatter is then completelyhydrolyzed by heating with water to produce good yields of thecorresponding arylphosphonic acid without a substantial loss ofphosphorus in the form of phosphoric acid or phosphorus oxychloride.

A second advantage of the present invention is that the phosphorussulfides do not catalyze the condensation of aromatic compounds such asnaphthalenes, alkylbenzenes and the like, and therefore this drawback ofthe earlier process is likewise substantially eliminated by the presentinvention.

The process is applicable in general to aromatic compounds that are freefrom polar groups capable of re acting with phosphorus sulfides such ashydrocarbons, e. g. benzene and its homologues, naphthalene and itshomologues, anthracene, phenanthrene; phenol ethers, e. g. anisole,phenetole. The ethers of monocyclic monohydric phenols react withespecial ease and give very high yields of the thionophosphine sulfidesand the phosphonic acids.

The reaction temperature will vary with the compounds; thus for example,some of the phenol others such as anisole react very smoothly andquantitatively at about the boiling point. Naphthalene gives bestresults at about l80 C. but on the other hand benzene and some of itshomologues such as o-xylene require temperatures of about 225 C. forbest results. In general with each compound it is desirable to operateat as low a temperature as possible in the range of optimumtemperatures, that is to say as low temperatures as produce reasonablyfast evolution of hydrogen sulfide. Higher temperatures in the case ofeach compound while still giving good yields are less desirable as thereis some effect on the yield and purity of the products obtained.

A very important, one might say very critical, requirement is that therebe a large excess of aromatic compound. In general the excess should beat least five moles of aromatic compound per mole of P 8 Larger ratiossuch as 10 or even 20 to 1 are preferable but, of course, after theoptimum results are obtained, further excess of aromatic compound, whileit does not adversely afiect the reaction, does reduce the output of theequipment, and therefore very large excesses over that required foroptimum results are not economically attractive.

The main reaction proceeds according to the equation Where Ar stands foraryl. However, side reactions take place in varying degrees with somearomatic compounds resulting in the production of lower sulfides ofphosphorus which are not reactive. In the case of ethers of monocyclic,monohydric phenols this side reaction is insignificant, but with benzeneand naphthalene it takes place to a considerable extent. When asparingly soluble lower sulfide of phosphorus is formed it crystallizestogether with the arylthiono-phosphine sulfide contaminating the latter.This is of little consequence if it is to be used only as intermediatein further reactions, e. g. in the hydrolysis to give the phosphonicacid or in the chlorination to give an aryltetrachlorophosphorane orarylphosphonothioic dichloride. However, it renders the preparation ofpure arylthionophosphine sulfides more difficult.

The prior art has described the reaction product of naphthalene with P 8as a compound having the formula Actually it is a mixture of 2- (not 1-)naphthylthionophosphine sulfide with a lower sulfide of phosphorus. Thisreference was badly misleading since a compound having the above formulawould give on hydrolysis not a monobut a di-phosphonic acid with thephosphorus group not in 2 but in 1 position.

In the phosphonation of naphthalene the side reaction can be suppressedto a large degree by using a very great excess of naphthalene (50 molfor 1 mol P S then the pure 2-naphthylthionophosphine sulfide can beisolated. Carrying out the reaction in a ratio of 10C H :lP S and adding40C H afterwards has not the same effect: the thionophosphine sulfide iscontaminated with a lower sulfide of phosphorus and does not crystallizein pure form. The large excess of naphthalene which is necessary toobtain the pure product has therefore not the function of a solvent forthe contaminant but is beneficial by its mass action.

However, the use of such a large excess of naphthalene is usually notnecessary for practical purposes where the contaminating lower sulfidesof phosphorus are either hydrolyzed or chlorinated to easily removablecompounds in subsequent reactions.

It is also possible to avoid the formation of lower sulfides ofphosphorus to a large extent by carrying out the phosphonation inpresence of some free sulfur which converts them to 113 Some of thearylthionophosphine sulfides, in particular p-anisylthionophosphinesulfide, tenaciously retain the solvent from which they have beencrystallized. It can be removed only by drying under reduced pressure atelevated temperatures. We are apparently dealing with rather stableclathrate compounds. Thus p-anisylthionophosphine sulfide crystallizedfrom anisole and dried in a vacuum desiccator at ordinary temperaturestill contained about 12% of anisole; after re-crystallization fromo-dichlorobenzene and washing with benzene it contained about 7% benzenewhich was even more tenaciously held than the anisole. While thisphenomenon is of no consequence in the further use of these products, itis of importance for determining their real yield, and for molecularweight determinations.

The molecular weight of p-anisylthionophosphine sulfide, determined on asample entirely freed from solvent, is that of the dimeric compound.Likewise the molecular weight determination of phenylthionophosphinesulfide corresponding to the dimer when the occluded solvent was takeninto account.

While these dimers are stable at ambient temperature, they undergofurther gradual polymerization on heating, forming brittle, transparentresins having probably the structure where stands for an indefiniteWhole number. In the case of p-anisylthionophosphine sulfide polymers ofthe formula Both the dimeric and the polymeric thionophosphine sulfidesare exceedingly sensitive to moisture which causes slow decompositionwith formation of hydrogen sulfide.

A most unusual behavior toward solvents is noted. Ordinarily a monomerand dimer is much more soluble than a relatively higher molecularpolymer. In the present case however. the dimer is almost completelyinsoluble in organic solvent whereas the polymers show fair to goodsolubility. While it is not intended to limit the invention to anyparticular theory we believe that the following is a possibleexplanation of the surprising behavior of the dimers and polymers towardsolvents.

Since a formula with a 4-membered ring 1K ft s s s is highly improbableit is assumed that the dimer is a salt-like compound formed bysemi-polar P-S bonds:

In the reaction of anisole with P 8 the latter attacks predominantly thenucleus. However to a very minor extent it attacks also the methoxygroup as evidenced by the formation of some methyl mercaptan when themother liquors from the crystallized thionophosphine sulfide arehydrolyzed.

From what has been said it becomes clear that it is advantageous toperform the reactions with P 3 at the lowest temperature at whichhydrogen sulfide evolution occurs and to discontinue the heating whenthis evolution ceases.

It is an advantage that a commercial grade of phosphorus pentasulfidecan be used. The use of a very pure grade offers but little advantage.The lower sulfides of phosphorus such as P 8 or P 8 do not phosphonatearomatic compounds such as anisole. When they are used together withsulfur, P 8 is formed and such mixtures are therefore chemicalequivalents of P 3 However there is no advantage in using such mixturesand the yields obtained with pre-formed P 5 are generally better. It isalso possible to use elemental phosphorus and sulfur but this does notoffer advantages.

The hydrolysis of the arylthionophosphine sulfides is a fairly slowreaction and prolonged boiling with water or heating under pressure isnecessary to obtain good yields of the arylphosphonic acids. It isnecessary to continue the hydrolysis until no more hydrogen sulfide isgiven off; blowing with steam to remove volatile and malodorousimpurities is not sufiicient. It is possible to effect hydrolysis underalkaline conditions. However, the

U reaction is much slower and in some cases where there are presentalkali soluble impurities an impure product is obtained. Hydrolysis byheating with water is therefore preferred.

After the hydrolysis is complete, the excess of aromatic compound mayreadily be stripped off with steam and the phosphonic acid recoveredfrom the aqueous solution. Where some by-products are formed as withsome of the aromatic compounds it is more advantageous to sepa rate theaqueous layer from the organic layer which contains contaminants. Insome cases the phosphonic acid is not sufficiently soluble in water andwill separate from the organic layer. In general the separation does notpresent any serious problem and therefore isolation of the firstcondensation product is not necessary. Where desired, however, suchisolation can be efiected because these reaction products are generallyinsoluble in the reaction medium and can be isolated by filtration andpurified by crystallization from appropriate solvents.

As has been pointed out the mechanism with different aromatic compoundsvaries and so does the yield of arylphosphonic acid from phosphorussulfide which varies from excellent with certain phenol ethers, such asanisole, to very good with other compounds. However, in each case theyields are superior to those obtainable by reaction of the aromaticcompound with hexagonal phosphoric anhydride.

During the reaction, particularly in the first stage, the presence oflarge amounts of oxygen is undesirable. In large scale equipment thefree air space, for example in suitable autoclaves, is so small that noserious precautions need be taken. However, in smaller equipment aninert gas atmosphere is preferable.

Some of the phosphonic acids which can be prepared according to thisinvention, for instance 2-isopropylnaphthylphosphonic acid, are of valueas surface active agents. Others are of use as intermediates in thepreparation of arylphosphonic dichlorides by conventional methods. Thesedichlorides in turn may be used for the preparation of esters to be usedas plasticizers or lubricating oil additives. Of particular interest arethe diallyl esters which give flame-resistant resins when polymerizedper se or co-polymerized with other monomers. (See J. Am. Chem. Soc.,vol. 70, page 186, and vol. 76, page 2191, and Ind. & Eng. Chem., vol.40, page 2276.)

The arylthionophosphine sulfides are useful intermediates for thepreparation of arylphosphonothioic dichlorides,aryltetrachlorophosphoranes and arylphosphonic dichlorides which in turnmay be converted into esters which are valuable insecticides.

The application is a continuation-in-partof our appli cation, Serial No.402,392, filed January 5, 1954, now abandoned.

The invention will be illustrated in the following specific examples inwhich the parts are by weight unless otherwise specified.

Example 1 175.8 parts of benzene and 48.8 parts of phosphoruspentasulfide are heated with agitation in a stainless steel autoclave at225 C. until reaction is complete. The clave contents consist of a tansolid and excess benzene. The solid is filtered oil. and washed withbenzene and dried. According to the analysis and the molecular weightdetermination in naphthalene it consists of a mixture of dimericphenylthionophosphine sulfide and P487 in the approximate molar ratio4:1.

The solid is heated with 200 parts of boiling water until hydrolysis iscomplete. The resulting solution is i clarified with decolorizingcarbon, concentrated to a small volume and cooled to produce a goodyield of phenylphosphonic acid.

This acid may be converted into its dichloride and further its diallylester by known methods. The use of this ester has been referred toabove.

Example 2 Example 3 POaHg 106 parts of o-xylene and 44.4 parts ofphosphorus pentasulfide are heated with agitation in a stainless steelautoclave at a temperature of 185 C. until the reaction is complete.After cooling, parts of water is added to the autoclave contents andrefluxing is carried out to hydrolyze a primary reaction product. Aseparation of the organic layer from the water layer is made. The waterlayer is clarified with decolorizing carbon and cooled to produce a goodyield of o-Xylylphosphonic acid; M. P. crude acid l49-152 C.,recrystallized from water 153-153.5 C. The homogeneous acid is probably3,4- dirnethylphenylphosphonic acid, but the position of the phosphonicgroup has not been proved.

The process was repeated attwo different temperatures, C. and 225 C. Ineach case good yields were ob tained of the xylylphosphonic acid but theyields were slightly lower than when the optimum temperature of C. wasused.

The acid is converted into its dichloride and into its diallyl ester byconventional means, the ester being useful in the production offlame-resistant resins.

Example 4 CHaOQPOaH;

432 parts of anisole and 177.6 parts of P 5 (molar ratio 10:1) areheated with stirring to reflux until the reaction is complete. Hydrogensulfide is evolved and the phosphorus pentasulfide gradually goes intosolution. Eventually some of the p-anisylthionophosphine sulfideprecipitates but its bulk crystallizes only on cooling,

forming a thick slurry. The solid is filtered off.

If a sample of the solid is washed with anisole only andthen dried in avacuum desiccator to constancy it still retains about 12% anisole. Thiscan be proved by heating the sample to 240 and distilling and collectingthe anisole. When a sample of the original solids is recrystallized fromo-dichlorobenzene and washed with benzene and dried in a vacuumdesiccator to constancy, it still retains about 7% benzene which can beremoved by heating to 240.

A sample of the original solid was dried under reduced pressure at 150until its weight became constant after 6 days. The analysis and amolecular weight determination in freezing p-nitrochlorobenzeneconfirmed the However, when the determination of the freezing point ofthe solution was repeated after several hours, the molecular weight hadincreased. This polymerization 7 was partly noticeable in moltennaphthalene. The meltmg point of the dimeric product is approximately225.

When the crystalline dimeric p-anisylthionophosphine sulfide is heatedin dry nitrogen just high enough to keep it liquid (about 210) forhours, polymerization occurs. After cooling there is obtained a hard,brownish, transparent, brittle resin, very soluble in most of the commoninert solvents.

The dimer is hydrolyzed by heating with eight times its weight of wateruntil hydrolysis is complete. By concentrating the aqueous solution thusobtained to a small volume and cooling, panisylphosphonic acid isobtained in excellent yield and purity, MQP. 179179.5 C. When thepolymeric resin was hydrolyzed in a similar fashion the yield ofphosphonic acid was considerably lower and a small amount of methylmercaptan was formed.

The above process was repeated at two diiferent temperatures, first at140 and then again at 200 C. Good yields were obtained though slightlylower than at the optimum temperature of 155160 C.

When, in the above process, P S +3S or P S +7S was substituted for P 8the reaction also proceeded but the yield of p-anisylphosphonic acid wassomewhat lower.

The dimeric p-anisylthionophosphine sulfide is useful as startingmaterial for insecticides. Treatment with 1 molar equivalent of chlorinein carbon tetrachloride gives p-auisylphosphonothioic dichloride. Thelatter can be converted in the conventional stepwise reaction into theethyl-p-nitrophenyl ester of p-anisylphosphonothionic acid, a veryefficient insecticide. In the form of a 1% dust the following percentagekills were obtained: mothweed bug 100%, confused flour beetle 95%, 0.1%feeding resulted in 100% kill of southern army worm and 95% kill ofAphis rumicis.

Example 5 256 parts of naphthalene and 44.4 parts of phosphoruspentasulfide are heated together with stirring at a temperature of 170C. until the reaction is complete. The temperature is lowered. Duringthis lowering the primary reaction product precipitates from the moltennaphthalene. Hydrolysis is accomplished without isolation by refluxingwith parts of water. Two layers result. The lower layer is separatedfrom the upper organic layer and traces of naphthalene are stripped fromthis lower layer. On cooling, 2-naphthylphosphonic acid precipitates ingood yield and purity.

The process was repeated at two other temperatures, first 150 C. andthen 210 C. In each case the yields were good but slightly lower thanthe optimum temperature of 170 C.

Z-naphthylphosphQnic acid is the starting material for the preparationof dyestuif intermediates, disclosed in the co-pending application by K.C. Whitehouse and H. Z. Lecher, Serial No. 361,088, filed June 11, 1953,now abandoned.

Thus, Z-naphthylphosphonic acid is sulfonated at low temperature bymixing 10.4 parts with 15 parts of 100% sulfuric acid and addinggradually 13.8 parts of oleum. The resulting5-sulfo-2-naphthylphosphonic acid is isolated as trisodium salt byconventional means. 12 parts of this salt fused with 15 parts of sodiumhydroxide and 3 parts of Water at 260320 C. give 5-hydroxy:2-naphthylphosphonic acid. The latter is coupled w1th tetrazotizeddianisidine to give a disazo dye which is recovered by salting out. itdyes cotton and rayon in blue shades from an alkaline solutioncontaining Glaubers salt.

Example 6 The pure primary reaction product of naphthalene withphosporus pentasulfide was isolated in two experiments. In the first onea very large excess of naphthalene was used. 2.5 moles of naphthaleneand 0.05 mole of P 3 were heated together with agitation at 170-180 C.until reaction was complete. The resulting solution was cooled to C. and360 cc. of benzene was slowly'added. The precipitate formed was filteredoff at about 50 C. and washed with benzene (M. P. 262.5265.5 C.). Afterrecrystallization from o-dichlorobenzene the product showed the meltingpoint 268- 271 C. and gave the correct analysis for the above formula.

In the second experiment a smaller excess of naphthalene was used, butsulfur was added. 1.0 mole of naphthalene, 0.1 mole of P 8 and 0.2 atomof sulfur were heated at l85190 C. until reaction was complete. Afterthe reaction mixture had been cooled to C. it consisted of a fine slurryto which there was added 250 cc. of benzene at a temperature of 7580 C.The crystalline product was filtered off at 5060 and washed withbenzene. After recrystallization from o-dichlorobenzene the product gavean analysis approaching the above sketched formula, but judging from theanalytical figures and its melting point, it was not quite as pure asthe product obtained in the first experiment.

( mOH,

parts of 2-isopropylnaphthalene and 44.4 parts of phosphoruspentasulfide are heated with stirring at a temperature of 170-175 C.until reaction is complete. The temperature is lowered to below 100 C.;then the reaction mixture is hydrolyzed by refluxing with 50 parts ofwater. The organic layer is separated and deposits on cooling the2-isopropylnaphthylphosphonic acid. The acid, recrystallized from analcohol-water mixture, melts at 21O4212 C. The position of thephosphonic group is not known. The product shows very good surfaceactive properties. The wetting of cotton yarn in dilute aqueous mediumwas almost instantaneous as compared to several minutes in distilledwater.

Example 8 CzHsOOPOsH? 122 grams of phenetole and 44.4 grams of P 8 (molar ratio 10:1) are heated to reflux with stirring until the reaction iscomplete. On cooling the p-phenetylthionophosphine sulfide crystallizesout and is filtered and washed with benzene. Its M. P. is approximately225. The hydrolysis carried out as in Experiment 4 gives an excellentyield of p-phenetylphosphonic acid.

We claim:

1. A process of producing an arylphosphonic acid which comprises heatingat least 5 molar equivalents of v 3. A process for producing anarylphosphonic acid which comprises heating at least 10 molarequivalents of an aromatic carbocyclic compound-free from polarsubstituents capable of reacting with P S at a temperature within therange of 140250 C. with 1 molar equivalent of P 8 hydrolyzing thereaction products by heating with water until evolution of hydrogensulfide substantially ceases, stripping simultaneously the excessaromatic compound ofi, and isolating the phosphonic acid from theaqueous still residue.

4. A process for producing a water-soluble arylphosphonic acid whichcomprises heating at least 10 molar equivalents of an aromaticcarbocyclic compound-free from polar substituents capable of reactingwith P 8 at a temperature within the range of 140-250 C. with 1 molarequivalent of P 1 hydrolyzing the reaction products formed by heatingwith water until the evolution of hydrogen sulfide substantially ceases,separating the 2 layers, and isolating the phosphonic acid from theaqueous layer.

5. A process for producing a water-insoluble arylphosphonic acid whichcomprises heating at least molar equivalents of an aromatic carbocycliccompound-free from polar substituents capable of reacting with P S at atemperature within the range of 140-250 C. with. 1 molar equivalent of P8 hydrolyzing the reaction product by heating with water until theevolution of hydrogen sulfide substantially ceases, separating thelayers, and isolating the phosphonic acid from the organic layercontaining the excess aromatic compound.

6. A process for producing an arylphosphonic acid which comprisesheating at least 10 molar equivalents of an aromatic carbocycliccompoundfree from polar substituents capable of reacting with P S at atemperature within the range of 140-250 C. with 1 molar equivalent of P8 isolating the'primary reaction prodnet by filtration, hydrolyzing itby heating with water until the evolution of hydrogen sulfidesubstantially ceases, and isolating the arylphosphonic acid.

7. A process according to claim 1 in which the aromatic compound isbenzene and the reaction temperature is within the range of 200-250 C.

8. A process according to claim 3in which the aromatic compound isbenzene and the temperature is within the range of ZOO-250 C.

9. A process according to claim 1 in which the aromatic compound iso-xylene and the temperature range is within the range of 160-2l0 C.

10. A process according to claim 1 in which the aromatic compound is anether of a monocyclic, monohydric phenol and the temperature is withinthe range of -200 C.

11. A process according to claim 10 in which the aromatic compound isanisole and the temperature is within the range of 140-200 C.

12. A process according to claim 1 in which the aromatic compound isnaphthalene and the temperature is within the range of 140-210 C.

13. A process according to claim 1 in which the aromatic compound is2-isopropylnaphthalene and the temperature range is 140210 C.

14. 2-isopropylnaphthylphosphonic acid.

15. A compound having the following formula (CHaO-O-ISa):

Loane et a1 Apr. 6, 1943 MacLaren Apr. 6, 1943

1. A PROCESS OF PRODUCIG AN ARYLPHOSPHONIC ACID WHICH COMPRISES HEATINGAT LEAST 5 MOLAR EQUIVALENTS OF THE CORRESPONDING AROMATIC CARBOCYCLICCOMPOUND, FREE FROM POLAR SUBSTITUENTS CAPABLE OF REACTING WITH P4S10 ATA TEMPERATURE FALLING WITHIN THE RANGE OF 140-250* C. WITH 1 MOLAREQUIVALENT 3F P4S10 AND HYDROLYZING THE REACTION PRODUCT BY HEATING WITHWATER UNTIL EVOLUTION OF HYDROGEN SULFIDE SUBSTANTIALLY CEASES. 14.2-ISOPROPYLNAPHTHYLPHOSPHONIC ACID.
 15. A COMPOUND HAVING THE FOLLOWINGFORMULA